Non-merohedrally twinned hexamethylenetetramine–4-nitrophenol–water (1/2/1), triclinic modification

The asymmetric unit of the title cocrystal, C6H12N4·2C6H5NO3·H2O, contains four formula units, which are linked by O—H⋯O and O—H⋯N hydrogen bonds into chains in the crystal. The crystal studied was a non-merohedral twin, with a minor twin component of 19%.


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For the background to this study and the C2 modification, see: Ng et al. (2001). The P1 modification is a non-merohedral twin; for the treatment of twinned diffraction data, see: Spek (2003).
on the phenolic oxygen atoms rather than on the ammonium nitrogen atoms as the compound displays hydroxyl absorption bands, as noted in the previous study (Ng et al., 2001).
The water H-atoms were placed in chemically sensible positions on the basis of hydrogen bonding but were not refined; their temperature factors were similarly tied. The anisotropic temperature factors of the water O-atoms were restrained to be nearly isotropic. Of the eight H-atoms, four of them are about 2 Å from another H-atom; nevertheless, these four form unambigously short hydrogen bonds with acceptor atoms.
From the reflections used in the initial indexing of the unit cell, the CELL_NOW routine of the APEX2 package (Bruker, 2007) had suggested the presence of a twin component, the routine giving the twin law as (-1 0 0, 0 -1 0, -0.313 0 1).

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The routine also suggested that the minor twin component was rotated from the principal component by 179.8° about the reciprocal axis along (0 0 1). The structure initially refined to an R index of 9.1% when twinning was not considered.